1. Field of the Invention
The present invention relates to a FM demodulation circuit, and more particularly to a FM demodulation circuit which can promote stabilization of circuit operation and does not need any adjustments and which can also obtain a demodulation output in the form of a pulse code modulated (PCM) signal.
2. Prior Art
As shown in FIG. 1, a conventional FM (frequency modulation) demodulation circuit is constituted by a comparator 1, a differentiation circuit 2, a monostable multivibrator 3 and a low-pass filter (LPF) 4. The comparator 1 compares a level of frequency-modulated (FM) wave with a reference level to thereby output a comparison result. The differentiation circuit 2 detects leading and trailing edges of an output signal of comparator 1 to thereby output a pulse signal. The monostable multivibrator 3 inputs this pulse signal outputted from the differentiation circuit 2 as a trigger signal to thereby output a pulse signal having the predetermined cycle. The LPF 4 filters out the low-frequency component signal from the pulse signal outputted from the monostable multivibrator 3. In the above-constituted FM demodulation circuit, the differentiation circuit 2 generates the trigger signal (shown in FIG. 2(c)) whose frequency is in proportion to the frequency of the inputted FM wave (shown in FIG. 2(a)). Hence, pulse density of the pulse signal outputted from the monostable multivibrator 3 will be in proportion to the frequency of the inputted signal thereof as shown in FIG. 2(d). Such pulse signal is converted into a smooth analog signal by the LPF 4 as shown in FIG. 2(e). Thus, the demodulation is completed.
In order to improve demodulation sensitivity in the above-mentioned conventional FM demodulation circuit, it is necessary to set optimum value between the pulse width of the pulse signal generated by the monostable multivibrator 3 and a carrier frequency of the FM wave. For this reason, there is a problem in that an adjustment of the pulse width is required and the stabilization of the pulse width becomes a problem.
Further, the low-pass filter 4 is required to have the characteristic which can sufficiently eliminate unnecessary higher harmonic wave components within the pulse train outputted from the pulse generating circuit (i.e., the monostable multivibrator 3). In addition, the resultant output signal of the conventional FM demodulation circuit must be in the form of an analog signal. Therefore, in the case where a latter stage (or latter-connected) circuit of this FM demodulation circuit needs digital processing, a band-limited LPF, a sample hold (S/H) circuit and the like must be required as an analog-to-digital (A/D) converter of twelve to sixteen bits having high accuracy and its peripheral circuits so that the cost will become higher.